In the fabrication of objects of many types and for many purposes, it has long been recognized that most moldable synthetic resins, at least those of low cost, have insufficient rigidity, strength or shock resistance to be capable of withstanding rough handling. Thus, while it is desirable to be able to fabricate packing cases and other structures for the handling, storage and transportation of commercial and industrial articles and goods-transporting palettes by molding from synthetic resin materials it has also been recognized that, especially in the case of such large objects, the properties of the plastic are insufficient.
Thus where the objects have good rigidity, strength, flexural characteristics and resistance to compression, it has often been found that the materials were nevertheless unsatisfactory because of poor shock resistance. For example, when these handling boxes and palettes were dropped or caused to impact against some more rigid objects, they frequently suffered from cracking. Experience has shown that there are few if any economical plastics capable of being molded into large objects such as goods-handling crates and palettes, which at the same time provide effective rigidity and shock resistance.
Thus it has been proposed to reinforce molded synthetic resin articles with armatures, skeltons or like structures of high rigidity, the armatures being constituted, for example, of wood or metal.
While such composite systems overcome the lack of ability of the plastic alone to resist shocks the standard practice of molding the synthetic resin material all around the armature so that the armature is completely sheathed in the synthetic resin material has proved to be disadvantageous.
More particularly, even with the most delicate selection of the synthetic resin material to be molded around armatures it is found that cracking of the sheath occurs, presumably as a result of shrinkage of the synthetic resin during the cooling of the product after molding the synthetic resin around the armature. This cracking, of course, can weaken the product or can cause separation of the sheath from the armature to the detriment of the product.